This invention relates generally to ultrasound systems, and more particularly, to control of ultrasound systems using Time Gain Compensation (TGC).
As an ultrasound wave travels through different layers of tissues, the amplitude of the wave decreases due to attenuation. Such energy loss is proportional to the tissue attenuation coefficient and the frequency of the ultrasound wave. Further, such energy loss also is proportional to the distance traveled by the ultrasound wave. Thus, echoes received from a tissue deeper in the body have less energy than those received from a similar tissue relatively closer to the surface of the body. For example, in Brightness mode (B-mode) grayscale imaging technique, the amplitude of a received echo is used to form an image. The brightness of a pixel in a B-mode image represents the strength of the corresponding echo. It is often desirable to display similar tissues with similar brightness, regardless of location relative to the transducer. TGC is often used in B-mode image processing to compensate for tissue attenuation in the body and equalize the tissue brightness across the entire image.
Several factors affect brightness for similar tissues, regardless of the position of the tissue relative to the transducer. Because the tissue attenuation coefficient varies among anatomies and body types (especially body with pathology) and the acoustic path an ultrasound wave travels within a body is often unpredictable, it is difficult to predict the exact attenuation an ultrasound wave experiences when traveling through the body. To compensate for energy loss due to attenuation, known ultrasound systems usually employ a combination of an internal TGC curve and external TGC sliding pots. The internal TGC curve is defined by predetermined values and is typically fixed for a given ultrasound system setting. These predetermined values may be calculated, for example, using statistical analysis on the acquired image data or empirically from an average body type. If, during a clinical scan, a patient has a body type that is very different from an average body type, the internal TGC does not properly compensate for the tissue attenuation. As a result, uneven brightness or banding artifacts may result in the displayed image. Therefore, manual adjustment of the external TGC sliding pots is performed to correct and/or minimize the banding. However, such adjustment is usually imprecise and time consuming. For example, because this adjustment requires human intervention, errors are more likely.
The banding artifacts are further visible when multiple focal zones are used to form an image. A focal zone is a location within the body at which the transmitted ultrasound wave is focused. Each focal zone has a corresponding focal region over which the energy transmitted to that focal zone produces the best image. Typically, different waveforms and frequencies of the ultrasound waves are used for different focal zones. The ultrasound image is obtained by combining multiple focal regions, wherein each focal region corresponds to a focal zone. Each focal zone may use different ultrasound waves, which tend to attenuate differently in the body. Thus, when two or more focal regions are joined together, the borders of the regions may be distinct and visible in the image as banding artifacts.